1. Field of the Invention
The present invention relates to a display device using a self-luminous element, and particularly to a display device using an electroluminescence element and a thin-film transistor.
2. Description of the Related Art
In recent years, electroluminescence (referred to hereinafter as "EL") display devices comprising EL elements are regarded as devices that may replace CRT and LCD. Research has been conducted for the development of EL display devices using, for example, thin film transistors (referred to hereinafter as "TFT") as switching elements to drive the EL elements.
FIG. 1 shows a plan view of a possible configuration of an active type EL display device, and FIG. 2 shows a cross-sectional view taken along line II--II of FIG. 1.
As shown in FIG. 1, a TFT is disposed near a junction of a gate signal line 51 with gate electrodes 11 and a drain signal line 52. The drain of the TFT is connected to the drain signal line 52, and the gate of the TFT is connected to the gate signal line 51. Further, the source of the TFT is connected to the anode 81 of an EL element.
As shown in FIG. 2, a display pixel 110 is formed by sequentially laminating a TFT and an organic EL element on a substrate 10 which may be a substrate made of glass or synthetic resin, a conductive substrate, or a semiconductor substrate. When a conductive substrate or a semiconductor substrate is used as the substrate 10, an insulating film made of materials such as SiO.sub.2 or SiN is deposited before the TFT is formed.
Gate electrodes 11 made of refractory metal such as chromium (Cr) are first formed on the insulator substrate 10. Subsequently, a gate insulating film 12 and an active layer 13 composed of p-Si film are sequentially formed.
In the active layer 13, channels 13c are formed in a position above the gate electrodes 11. Ion doping is performed using stopper insulating film 14 formed on the channels 13c as masks. Regions on both sides of the gate electrodes 11 are then covered with a resist, and further ion doping is conducted. As a result, low-concentration regions 13LD are disposed on both sides of the channels 13c. Furthermore, a source 13s and a drain 13d, which are high-concentration regions, are formed on the outer sides of the low-concentration regions 13LD, respectively. The described structure is the so-called LDD (Lightly Doped Drain) structure.
Subsequently, an interlayer insulating film 15 comprising a sequential lamination of a Sio.sub.2 film, a SiN film, and a SiO.sub.2 film is formed to cover the entire region over the gate insulating film 12, the active layer 13, and the stopper insulating film 14. A contact hole formed with respect to the drain 13d is filled with metal such as aluminum (Al) to provide the drain electrode 16. A planarizing insulating film 17 consisting of, for example, organic resin is formed over the entire surface, planarizing the surface. A contact hole is formed in the planarizing insulating film 17 at a location corresponding to the source 13s, and an anode 81 of an EL element is then formed over the planarizing insulating film 17 through the contact hole. The anode 81 is composed of ITO (Indium Tin Oxide), and simultaneously serves as the source electrode through its contact with the source 13s via the contact hole.
Subsequently, an EL element is formed on the anode 81.
In the organic EL element, holes injected from the anode 81 and electrons injected from the cathode 87 recombine in the emissive layer which is one layer within the emissive element layer 86 including organic compounds. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the side of the transparent anode 81 via the transparent insulator substrate 10, resulting in light emission.
When the EL element is formed as described above, the emissive element layer 86 deposited over the anode 81 is extremely thin, generally at a thickness of approximately less than 2000 .ANG.. The emissive element layer 86 therefore provides poor coverage at uneven portions on the planarizing insulating film 17 at the peripheral portions of the anode 81 (indicated by arrows). The poor coverage may also result at irregularities in the surface created by the TFT through, for example, the thickness of the Al wiring. A problem exists where the emissive element layer 86 becomes disconnected, and the cathode 87 disposed over the emissive element layer and the anode 81 forms a short circuit at the disconnected portion. In such cases, pixels produce deficient displays.
Another existing problem is that an electric field becomes concentrated at the uneven portions created by the thickness of the anode 81, especially at the edges on the peripheral portions of the anode 81, thereby speeding up the deterioration of the emissive layer.
A further disadvantage is that some of the emitted light irradiates the TFT underneath the emissive element layer. Because of such light, leakage current of the TFT increases, and stable TFT characteristics and stable display cannot be achieved.